If you feel mentally foggy, sluggish, forgetful, or not quite as focused as usual, you may attribute it to lack of sleep or daily stress. What may not occur to you is that the cause could be simpler—and at the same time more serious: a deficiency in one essential nutrient.
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Vitamin B12, which often does not receive much attention, plays a central role in the functioning of the nervous system and brain health. It helps support memory, concentration, and mood, and contributes to maintaining the integrity of nerve cells. When B12 levels decline in the body, the brain is among the first organs to be affected, leading to symptoms such as mental fog, memory problems, and reduced ability to concentrate.
Vitamin B12 deficiency is more common than many people realize. Estimates suggest it affects about 6% of Americans under the age of 60 and around 20% of those over 60, though exact figures vary by study.
Several factors contribute to this deficiency, including diet quality, medications, absorption problems, and aging, with older adults being particularly vulnerable.
Dr. Majd Fattouh, neurologist and assistant professor at Johns Hopkins University, says:
“Vitamin B12 deficiency is one of the few reversible causes of cognitive decline. Detecting it early can prevent long-term brain damage and restore mental clarity.”
So what should you know about vitamin B12 and brain health? Why is it so essential, how can you get enough of it, and what signs may indicate a deficiency?
How Does Vitamin B12 Support Brain Health?
Vitamin B12 affects brain health in multiple interconnected ways. It plays a key role in building and maintaining the myelin sheath—the protective covering surrounding nerve fibers that allows brain cells to communicate quickly and efficiently. It is also essential for DNA synthesis and the long-term health of brain tissue.
Beyond that, B12 is necessary for the production of neurotransmitters such as dopamine and serotonin, vital chemicals that regulate mood, focus, and motivation.
Another important role of vitamin B12 is helping the body break down homocysteine, an amino acid that can accumulate when B12 levels are low. Elevated homocysteine has been linked to increased oxidative stress and impaired blood vessel function, which may worsen cognitive decline and negatively affect brain function.
Dr. Fattouh explains:
“Taken together, these roles make vitamin B12 essential for clear thinking, strong memory, and mental energy.”
When B12 levels drop, nerve signaling may slow or become disrupted, making brain cells more vulnerable to damage. Over time, deficiency can lead to confusion, balance problems, slowed thinking, and difficulty concentrating.
Some research has also linked high homocysteine levels to cognitive decline and an increased risk of neurodegenerative diseases such as Alzheimer’s and other forms of dementia.
Signs of Vitamin B12 Deficiency
When vitamin B12 levels fall, symptoms may affect multiple body systems. Common signs include fatigue, exhaustion, and general weakness due to anemia, as well as numbness or tingling in the hands and feet, heart palpitations, memory problems, difficulty concentrating, balance issues, and mood changes such as irritability, depression, and apathy.
Dr. Brian J. Copeland, a neurologist at University Medical Center in New Orleans, notes that anemia is common in people with B12 deficiency, but neurological symptoms may appear even in the absence of anemia when B12 levels are low.
Symptoms often develop gradually and may be subtle at first, but they tend to worsen over time if the condition is not diagnosed and treated.
Nutritionist Patricia Kolesa says:
“Symptoms of vitamin B12 deficiency may not appear until levels become significantly low.”
A blood level of 200 picograms per milliliter or less is considered a clear indicator of deficiency. Measuring homocysteine and methylmalonic acid can also provide a clearer picture of B12 activity in the body.
In a recent study, some older adults showed cognitive and neurological symptoms even though their B12 levels were technically within the normal range, but at the lower end.
If you are concerned about your memory, experience fatigue, weakness, balance problems, or any of the symptoms mentioned, consulting a physician or healthcare professional and testing vitamin B12 levels may be an important step in protecting brain health and preventing complications.
Dr. Joel Salinas, clinical associate professor of neurology at NYU Grossman School of Medicine, says:
“Vitamin B12 deficiency is common and often overlooked, yet it is relatively easy to diagnose and treat. Because its neurological and cognitive effects can resemble more serious brain disorders, it is routinely tested for during evaluations of cognitive decline.”
Risk Factors for Vitamin B12 Deficiency
Vitamin B12 is essential for red blood cell production. To maintain adequate levels, a person must consume foods containing B12 and be able to absorb it properly. A protein called intrinsic factor, produced by stomach cells, plays a key role in absorption.
The body cannot produce vitamin B12 on its own. Therefore, inadequate dietary intake, lack of supplementation, or absorption problems can all increase the risk of deficiency.
Nutritionist Patricia Kolesa explains:
“Vegetarians and those following a strict vegan diet are at higher risk of vitamin B12 deficiency, as this vitamin is found primarily in animal-based foods.”
People with gastrointestinal conditions that impair absorption—such as Crohn’s disease, celiac disease, pernicious anemia, a history of stomach or bariatric surgery—as well as those who consume excessive alcohol, are also at risk. Older adults are particularly vulnerable, as the body’s ability to absorb B12 declines with age.
Dr. Salinas adds that individuals taking metformin or long-term acid-reducing medications such as proton pump inhibitors may also develop B12 deficiency and should be monitored regularly.
How Is Vitamin B12 Deficiency Treated?
Following a diet rich in vitamin B12 helps maintain healthy levels. Key dietary sources include fish, meat, eggs, dairy products, nutritional yeast, and fortified breakfast cereals.
Dr. Salinas says:
“Oral vitamin B12 supplements are effective for most patients, while injections are used in more severe cases. Blood testing is the most reliable way to determine whether diet alone is sufficient to meet the body’s needs.”
Early screening is crucial to detect vitamin B12 deficiency before it reaches a stage that is difficult or impossible to reverse.
When treated early, memory problems, mood changes, and concentration difficulties can resolve completely. Without treatment, permanent memory loss may occur, according to Dr. Brian Copeland.
He adds:
“In more severe cases, the condition may progress to overt dementia with behavioral symptoms, including psychosis. This highlights the importance of checking vitamin B12 levels in anyone experiencing unexplained memory decline.”
If a true deficiency is present, patients may need oral or injectable vitamin B12 supplements, depending on the case and medical assessment. A primary care physician can evaluate vitamin B12 levels with a simple blood test to determine whether a deficiency or low level exists.
